1. Field of the Invention
The present invention relates to technology for negotiating a connection identifier when a signal is issued (when a call connection request is issued) in a switched virtual channel (SVC) service, etc. of an asynchronous transfer mode (ATM) system.
2. Description of the Related Art
The technology for negotiating for a connection identifier when a signal is issued (when a call connection request is issued) in an SVC service, etc. of the ATM system can be one of the following two methods. That is, one method is for determining a connection identifier by a network, and the other method is for determining a connection identifier by a terminal unit.
FIG. 1 shows the conventional sequence in which the connection identifier is determined by the network.
First, a source ATM terminal 1 (A1) sends a request to set up a connection to a signaling unit in terminating equipment (S1).
Next, the source signaling unit sends a SETUP message (call setup message) to a network (S2). In this case, no conditions are specified in the SETUP message for the value of the connection identifier (C-ID).
When the network receives the SETUP message, it determines a connection identifier corresponding to the connection requested by the message, generates another SETUP message which specifies the value of the connection identifier, and sends it to a signaling unit in the terminating equipment at the destination terminal (S3). In response to the SETUP message received from the source terminal, the network returns a CALL-PROCEEDING message (CALL-PROC message) which specifies the value of the above described connection identifier to the signaling unit at the source terminal (S4).
When the signaling unit at the destination terminal receives a SETUP message from the network, it unconditionally accepts the value of the connection identifier specified by the SETUP message, and sends a connection setup notification to a destination ATM terminal 2 (A2) (S5). The signaling unit returns a CONNECT message (answer message) to the network (S6).
When the network receives the CONNECT message from the signaling unit at the destination terminal, it returns a CONN-ACK message (acknowledgement message) to the signaling unit (S7) and returns the CONNECT message to the signaling unit at the source terminal (S8).
When the signaling unit at the source terminal receives the CONNECT message after the CALL-PROC message from the network, it returns the CONN-ACK message to the network (S9). In this case, the signaling unit unconditionally accepts the value of the connection identifier specified by the CALL-PROC message.
As shown in FIG. 1, when the connection identifier is determined by the network, the network completely controls the determination of the connection identifier while the terminal unconditionally accepts the value of the connection identifier determined by the network.
FIG. 2 shows the conventional sequence when the connection identifier is determined by the terminal.
First, the source ATM terminal 1 (A1) sends a request to set up a connection to a signaling unit in a terminating equipment (S1).
Next, the source signaling unit sends a SETUP message (call setup message) in which conditions for the value of a connection identifier (C-ID) are specified to a network (S2).
When the network receives the SETUP message, it determines the value of a connection identifier based on the conditions for the value of the connection identifier specified by the SETUP message, generates another SETUP message which specifies the value of the connection identifier, and sends it to a signaling unit in the terminating equipment at the destination terminal (S3). In response to the SETUP message received from the source terminal, the network returns a CALL-PROCEEDING message (CALL-PROC message) which specifies the value of the above described connection identifier to the signaling unit at the source terminal (S4).
When the signaling unit at the destination terminal receives a SETUP message from the network, it determines whether or not it accepts the value of the connection identifier specified by the SETUP message, and sends a connection setup notification to a destination ATM terminal 2 (A2) (S5). The signaling unit returns a CONNECT message (answer message) to the network (S6). When the signaling unit does not accept the value for the connection identifier specified by the received SETUP message, it specifies an amended value for the connection identifier in the above described CONNECT message.
When the network receives the CONNECT message from the signaling unit at the destination terminal, it returns the CONN-ACK message (acknowledgement message) to the signaling unit (S7) and returns the CONNECT message to the signaling unit at the source terminal (S8).
When the signaling unit at the source terminal receives the CONNECT message after the CALL-PROC message from the network, it returns the CONN-ACK message to the network (S9).
If a connection identifier is determined by the terminal unit as shown in FIG. 2, the conditions for the connection identifier should be determined at the terminal unit.
The limitations on the hardware or software may limit the range of each value of the connection identifier supported by the ATM terminal unit :
virtual path identifier (VPI) and virtual channel identifier (VCI). Practically, the following limitations may be placed.
1. limitation on VPIs only PA1 2. limitation on VCIs only PA1 3. limitation on VPI and VCI PA1 000: VPCI is invariable, and VCI is also invariable. PA1 001: VPCI is invariable, but VCI is variable. PA1 Others: reserved values
These limitations may cause the following problems in the conventional technology.
When a connection identifier is determined by the network as shown in FIG. 1, the network entirely controls the determination of the connection identifier. Therefore, the network should be informed of the limitations on the range of the value of the connection identifier at each ATM terminal unit. Conventionally, it is required that the limitations should be entered off-line for each ATM terminal unit, and the entry operations are complicated and errors can easily be made.
If the connection identifier is determined by the terminal unit as shown in FIG. 2, limitation 2 (on VCIs only) of the above listed three limitations cannot be placed. FIG. 3 shows the data format for use in specifying the connection identifier contained in the conventional signaling message. In FIG. 3, a VPCI (corresponding to the VPI) is stored in the field "virtual path connection identifier", and a VCI is stored in the field "virtual channel identifier". The invariable indication field "preferred/exclusive" contains the following 3-bit values.
When the invariable indication field is assigned the value 000 (both VPCI and VCI are invariable), the ATM terminal unit has to manage the use states of both VPCI and VCI. When the invariable indication field is assigned the value 001 (the VPCI is invariable and the VCI is variable), the ATM terminal unit has to manage the use states of the VPCI. In designing the ATM terminal unit, a configuration in which only the VCI has to be managed may be required. However, the conventional technology has the problem that it cannot design an ATM terminal unit to satisfy this requirement.